Oil prices have a long history of violent swings. But even by those standards, crude just experienced an incredibly extreme move.

Flash forward to today and oil is racing back to nearly $50 a barrel. That amounts to a 30% spike in the span of merely a week. Over the past three days alone crude enjoyed its largest percentage gain since August 1990, according to The Wall Street Journal.

So what changed so dramatically? Fundamentally, not all that much. It still looks like the world has too much oil, thanks to the gush in production created by the American shale oil boom.

Concerns about a supply glut may have been eased a bit on Monday with a new government report that downgraded its estimate for U.S. oil output so far this year.

But that report hardly justified the surge in oil prices that followed. And U.S. oil production is still up from a year ago.

Oil prices were also boosted by hope that OPEC may finally be willing to cut back on output in an effort to balance the market. It was fueled by an OPEC publication released Monday that reiterated that the oil cartel "stands ready to talk to all other producers."

Matt Sallee, a portfolio manager at Tortoise Capital, pointed to a bullish report showing crude stockpiled declined sharply last week, the stronger-than-expected U.S. economic growth for the second quarter and more geopolitical tension between Saudi Arabia and Yemen.

"Oil prices below $50 are not sustainable for the long term. I think everyone realizes that," Sallee said.

The rebound has been great news for beaten-down energy stocks. Shares of ConocoPhillips(COP), Consol Energy(CNX) and Chesapeake Energy(CHK) all rallied sharply higher on Monday.